It should be based on series of these articles:

http://mswiderski.blogspot.com/2015/12/kie-server-extend-existing-server.html

I can provide some more examples / tutorial if needed

context:
kie-server extensions is very powerful technique for extending our product in a documented, supported, way. Sometimes customers are requesting features which we lack, and one way is to raise RFE - while this is a standard workflow, sometimes, it takes years for RFE to get incorporated into the product. If customer would built kie-server extension it's matter of days/weeks.

The scope of support is similar as with Work Item Handlers - we do support the integration points, we do not support the custom code itself.

(Addition from sterobin@redhat.com)

Target users: KIE Server administrator and rules/process developer. See [https://drive.google.com/drive/u/1/folders/0B6cY6hNQRMMTSzJsVENXR0N4TFU|persona profiles] for details.

Squad:

• swiderski.maciej (eng)
• mmacik@redhat.com (QE)
• rhn-support-agiertli (support, reporter)

Resources:

• This is the parent article, which links to the three relevant "Extending" articles. All four are great sources for this doc.
• Legacy Dev Guide equivalent to the REST extension article specifically.
• KIE Server Overview and Glossary in jBPM community doc, briefly describing extensions. May be useful in conceptual introductory content.

Things to also consider including:

• Maciej's article on custom form rendering. It's related to extending the KIE Server functionality.